This invention relates to double-axis micromirrors such as MEMS arrays and hinges therefore used with parallel plate electrostatic actuation.
Micromachined double-axis micromirrors are used in communication for optical components, such as optical switches, attenuators, and modulators. These mirrors typically include either circular or square flat reflective surfaces fabricated of either single-crystal silicon or polysilicon. The mirrors are attached to a surrounding support region by the use of either a flexure or a double gimbaled structure whereby a gimbal ring is connected to the support structure by a pair of hinge elements along an axis bisecting the support ring, and the gimbal ring is coupled to a mirror within the gimbal ring by a pair of second hinge elements along a second axis bisecting the ring and transverse, preferably orthogonal, to the first axis. The purpose of the flexure and/or hinge elements is to allow free double-axis movement of the mirror.
The electrostatic actuation methods of micromachined mirror arrays generally fall into two categories; those involving parallel-plate electrostatic actuation and those involving comb-drive electrostatic actuation. In the case of comb-drive actuation, comb-drive actuators develop forces between interdigitated combs that are located outside the boundary of and away from the mirror elements. The actuators are connected to the mirror by the use of linkage elements which are typically in contact with each other. Advantages of this technique are that the electrostatic forces in question are decoupled from the mirror design, allowing conceivably lower voltages for a given force. Disadvantages are difficulty in designing double-axis mirrors so they can reside in a compact region of space, difficulty in manufacturing, difficulty in interconnection, and potential for undesired contact with adjacent components and regions, which is a known serious reliability issue for MEMS devices.
In the case of electrostatic parallel plate actuation, forces are developed between the mirror and one or more electrodes embedded in the substrate. The electrostatic forces applied to the mirror are purely attractive. Because there are no repulsive forces on the mirror, the same forces that contribute to mirror rotation (which is desirable) also contribute to a vertical sag component of the mirror (which is undesirable). Advantages of micromachined electrostatic mirrors that utilize parallel-plate actuation are ease of design, ease of manufacture, and ease of compact interconnection, i.e., such mirrors can be deployed with efficient fill factors. In most cases, these designs are also non-contact in that the actuation element is not directly coupled to the mirror. A disadvantage is that the electrostatic force applied to the mirrors is interactive with the mirror design. This can either limit resonant frequencies (and therefore switching times), limit maximum tilt angles, or dictate higher design voltages.
A micromachined mirror may be connected to a surrounding support structure by means of either isolated flexure elements or hinge elements in combination with a gimbal which surrounds the mirror. Flexure elements can eliminate the need of a gimbal structure. The number of flexure elements is not necessarily restricted by design, since the rotation of the mirror is given by overall flexure compliance.
One example of a flexure support structure without gimbals was built by Dr. Li Fan, currently associated with by Optical Micro-Machines, Inc. (OMM) of San Diego, Calif. In this structure, four single-beam flexure elements are connected at four corners of a square mirror to the surrounding support region and extended along adjacent edges of the mirror. The flexure elements operate in a leaf spring mode. A major drawback of this structure is that the same type of flexure beam bending that accomplishes rotation also contributes to sag. Because electrostatic forces are only attractive, as the mirror rotates, it will undergo sag, reducing the amount of overall possible rotation for a given design.
Another type of micromachined double-axis mirror utilizes a gimbal. In this configuration, a micromachined mirror is connected to a surrounding support region by means of hinge elements in combination with a gimbal which surrounds the mirror. Each rotational axis is connected to the gimbal by two opposing hinge structures, for a total of four rotational hinge structures.
An example of a gimbal in combination with four rotational hinge structures has been developed by Lucent Technologies and Agere Systems Optoelectronics Guardian Corp. and described in U.S. Pat. No. 6,265,239B1 issued Jul. 24, 2001. In this example, two hinge elements per degree of freedom are connected between the mirror and the gimbal, and provided with a multiply-meandered transverse hinge structure. The transverse meanders are aligned so that the long axis of the hinge elements are transverse, or perpendicular to the axis of rotation. Rotation is accomplished by each of the hinge elements bending along the long direction in small paraboloidal sections. As in the flexure support structure, a significant drawback of this design is that the same beam movement which supplies rotational motion also supplies a sag and lateral translation element. Because of this typical designs utilizing transverse meandered hinges have the drawback that for some design spaces, the movement can be significant. This may either limit the maximum rotation angle of the mirror, or create an undesirable piston mode of oscillation which may interfere with the fundamental torsional modes of oscillation.
U.S. Pat. No. 6,044,705 shows a torsional bar design suitable primarily for a larger mirror array than of interest. However, it fails to recognize or address problems which occur in compact structures where the size of the hinge and mirror are impacted by forces and factors consistent with electrostatic actuation forces.
Other patents concerning double-gimbaled micromachined mirrors include 5,488,862; 5,648,618; 5,895,866; 6,122,394; 6,060,479; 6,044,705; 5,969,465; 5,629,790; 5,841,533; 5,016,072; 4,317,511; and 4,598,585. These patents have been listed for completeness, but they are not believed to be directly relevant prior art.
It is difficult to determine the proper combination and values of factors that can be optimized and compromised in order to produce a suitable size of an array of mirrors and a suitable mirror size. The problem is thus one of determining a design space which optimizes all relevant considerations. Factors to be included are primary torsional spring constants, secondary torsional spring constants, vertical and lateral linear spring constants (for shock considerations), hinge compactness, manufacturability, and process dependent tolerancing.
As an illustration of a hinge structure which is believed to be of the prior art but which does not meet all desired criteria, reference is made to FIGS. 9, 10 and 11. In FIG. 9, a torsional hinge structure is shown in the form of a transverse serpentine structure. The transverse hinge structure has a plurality of serpentine hinge elements 90-93 linked together by rigid braces 99, 101-106. However, the hinge elements are orientated perpendicular to the axis of rotation 114. This has several important consequences in design. First, overall rotation is accomplished only by parabolic beam bending along the length of the entire beam. This is understood by examining FIG. 10. Connection point 112 is fixed, and an external torque is applied at connection point 110. Rotation is accomplished as elements 90-93 bend into opposing paraboloidal sections as demonstrated in FIG. 11, which is a side view of FIG. 10 with exaggerated rotation. The effective torsional spring constant for this structure is given by the expression:       k    θ    =            Ewt      3              6      ⁢              NL        ⁡                  (                      1            -                          v              2                                )                                    where        kp is effective transverse torsional spring constant,        w is hinge width,        L is hinge length,        N is number of hinge elements, and        E is Young's modulus.        
An important consideration is that the same motion that contributes to sag also contributes to rotation, i.e., beam bending along the length of each of the hinge elements. This has important consequences in device performance, as electrostatic forces which give rise to adequate movement for rotation will also give rise to significant sag.
What is needed is a manufacturable flexure element that is useable as a hinge for a miniature steerable mirror that allows for a compliant torsional rotation while simultaneously restricting net vertical and horizontal displacement.